Intermittent Old Push Button Type MCB Tripping


Postby garfield59 » Sun Mar 08, 2015 2:43 pm

About a month or so ago our ring main MCB tripped for no apparent reason. I reset it and since then it trips on a random basis, mostly in the early hours of the morning but not at times when you might expect it to if there was a loading problem e.g. when the tumble drier, toaster kettle are all doing their stuff.
Most of the time when it trips the only things switched on are fridge, freezer i.e. the usual "permanent" items.
There are no signs of appliances or sockets not working between spurious trips so I don't think there is permanent cable damage anywhere.
I am now wondering if the MCB is faulty because it's an old push button type of at least 20 years vintage.
We will be getting a re-wire later this year when we have an extension built but I don't want to hang around for a solution until then because randomly having to reset the MCB is starting to get on my nerves!
Any advice welcome, thank you
Garfield
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Postby ericmark » Mon Mar 09, 2015 7:08 am

You can still buy the push button MCB but no longer has a push button at £10 that may be the easy way http://www.screwfix.com/p/wylex-32a-sp- ... -mcb/87979 but it is unusual for a MCB to fail as such.

With RCD tripping we often look to frost free freezers as these have heaters which do the defrost which will switch on at random times and when faulty can cause a RCD to trip at random times. However freezers don't tend to have 13A fuses tends to be 5A and one would expect the fuse to blow rather than trip a MCB.

A 32A B type MCB is two units in one to trip the thermal part needs 32A for a long time but to trip the magnetic part needs 160A for around 0.04 seconds so if the heater was short circuit it would likely trip a 32A MCB before a 13A fuse would blow but it has to be a short circuit not just an over load.

So at £10 since simple plug in units the swapping MCB is the simple first step.
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Postby garfield59 » Tue Mar 10, 2015 4:04 pm

Thank you for the reply. I will get the replacement ASAP and let you know if it sorts it.
No trips since Sunday even though we have been using all the usual money-burners such as washing machine and tumble drier.
Will report back soon.
Garfield
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Postby garfield59 » Tue Apr 14, 2015 9:12 pm

Well I finally replaced the MCB last weekend after a month or so where we had no trips for about four weeks and then last weekend we had two on Saturday and two on Sunday. Since changing the unit we have had no trips plus we may have found the offending electrical equipment - an extension socket in my daughters room with what seems to be a loose connection inside it. It is no longer in use!
Time will tell if we have solved the problem - I will keep you posted.
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